Construction machine

ABSTRACT

An object of the present invention is to provide a construction machine that is capable of effectively charging electrical power into the capacitor in consideration of the drive of a swing section and hydraulic work device. To achieve the object, the present invention grasps the status of the construction machine ( 2 ) in advance to calculate energy, predicts the next operation, determines the amount of energy to be charged into a capacitor ( 13 ), and controls a DC-DC converter ( 15 ) accordingly. Consequently, the loss in a conventional battery (lead storage battery) during regeneration can be reduced to provide increased efficiency.

TECHNICAL FIELD

The present invention relates to a construction machine having an engineand a power supply system.

BACKGROUND ART

In the past, construction machines such as a hydraulic excavator drove ahydraulic pump with the output torque of an engine to drive a hydraulicwork device (a bucket cylinder, an arm cylinder, or a boom cylinder).

In recent years, a hybrid construction machine having, for example, anengine, a power generator (motor-generator), an electric motor (ACmotor), and an electrical storage device (battery) has been proposed inorder to provide improved fuel economy, reduce emissions, and reducenoise.

For example, the battery is charged under light load conditions withelectrical power generated by the power generator and with regenerativepower obtained from braking, and the charged electrical power isdischarged from the battery and used under heavy load conditions.

Especially in the case of a hybrid construction machine that uses a leadbattery, the lead battery is not efficiently charged with theregenerative power obtained from braking because the regenerationefficiency of the lead battery is low.

A technology described, for instance, in JP-A-1997-224302 relates to ahybrid vehicle and provides improved regeneration efficiency. Morespecifically, the regeneration efficiency is improved by configuring apower supply system having a lead battery and a capacitor, using thecapacitor to store energy, and supplying the stored energy to a load.The technology described in JP-A-1997-224302 also detects the speed ofthe vehicle and changes the target electrical storage ratio of thecapacitor in accordance with the vehicle speed for the purpose ofexercising control to increase the target voltage of the capacitor whenthe vehicle speed is low and decrease the target voltage of thecapacitor when the vehicle speed is high.

Another technology described, for instance, in JP-A-1999-164402 selectsa relatively high charge state for an electrical storage section whenthe speed of a hybrid vehicle is low and selects a relatively low chargestate for the electrical storage section when the speed of the hybridvehicle is high.

Still another technology described, for instance, in JP-A-2002-359935relates to a hybrid work machine and appropriately changes acharge/discharge threshold value for an electrical storage section. Morespecifically, this technology selects a low charge/discharge thresholdvalue for the electrical storage section when kinetic energy andpotential energy are high and selects a high charge/discharge thresholdvalue for the electrical storage section when the kinetic energy andpotential energy are low.

PRIOR ART LITERATURE Patent Documents

-   Patent Document 1: JP-1997-224302-A-   Patent Document 2: JP-1999-164402-A-   Patent Document 3: JP-2002-359935-A

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In general, a vehicle is electrified by electrifying its travel section.

However, when a construction machine is to be electrified, it involvesnot only its travel section but also a system that drives a swingsection and a hydraulic work device by using hydraulic pressure or byusing electrical power stored in a battery or in a capacitor.

More specifically, the construction machine is electrified by using anAC motor to drive a swing structure and by using a motor-generator toassist the drive of a hydraulic pump for driving the hydraulic workdevice.

As described above, when the construction machine is to be electrified,the target of electrification is not limited to its travel section.Further, charge/discharge control for the battery and for the capacitoris not simple, but the drives of the swing section and of the hydraulicwork device need to be taken into consideration.

Moreover, in a system that uses both the capacitor and the battery, thecapacitor needs to be effectively charged with electrical power.

The present invention has been made in view of the above circumstancesand provides a construction machine that is capable of effectivelycharging the capacitor with electrical power in consideration of thedrive of the swing section and hydraulic work device.

Means for Solving the Problem

According to one aspect of the present invention, there is provided aconstruction machine including an engine, a hydraulic pump, amotor-generator, a hydraulic work device such as a bucket cylinder, anarm cylinder, or a boom cylinder, a swing structure, an AC motor, and apower supply system, and a controller. The hydraulic pump is driven bythe engine. The motor-generator is coupled to the engine and to thehydraulic pump and capable of generating electrical power. The hydraulicwork device is driven by a hydraulic fluid discharged from the hydraulicpump. The swing structure is a structure on which the hydraulic workdevice is mounted. The AC motor drives the swing structure. The powersupply system includes a capacitor and battery that supply electricalpower to the motor-generator and/or the AC motor (preferably the ACmotor) and become charged with electrical power regenerated by themotor-generator and/or the AC motor (preferably the AC motor). A leadbattery or a lithium-ion battery may be used as the battery. However, itis particularly preferred that a lead battery be used as the battery.The controller controls the power supply system.

It is preferred that the controller include an motion estimationsection, an energy calculation section, and a capacitor control section.The motion estimation section estimates the next motion of the hydraulicwork device and/or the swing structure (preferably the next operation ofthe swing structure) in accordance, for instance, with the currentoperation by an operator and with measured load. The energy calculationsection calculates the amount of electrical power to be regenerated bythe motor-generator and/or the AC motor (preferably the AC motor) inaccordance with the estimated next operation. The capacitor controlsection sets a target voltage for the capacitor in accordance with thecalculated amount of electrical power.

It is preferred that the controller include a body control section thatcalculates the amount of electrical power to be requested from thehydraulic work device and/or the swing structure (preferably the swingstructure) in accordance with the current operation by the operator.

It is preferred that the capacitor control section include a capacitorstatus detection section, a capacitor charge/discharge commandcalculation section, and a capacitor target voltage setup section. Thecapacitor status detection section detects the current voltage of thecapacitor. The capacitor charge/discharge command calculation sectioncalculates a charge/discharge command for the capacitor in accordancewith the calculated amount of electrical power and with the detectedcurrent voltage. The capacitor target voltage setup section sets atarget voltage for the capacitor in accordance with the calculatedcharge/discharge command.

It is preferred that the capacitor control section be capable ofcorrecting the target voltage for the capacitor in accordance with therotation speed of the engine, a torque command for the AC motor, and/ora torque command for the motor-generator.

According to another aspect of the present invention, there is provideda construction machine including a power supply system and a controller.The power supply system includes a capacitor and battery that supplyelectrical power to an AC motor for driving a swing structure and thatbecome charged with electrical power regenerated by a motor-generatorand/or by the AC motor. The swing structure is a structure on which themotor-generator and/or a hydraulic cylinder are mounted. Themotor-generator is coupled to an engine and to a hydraulic pump andcapable of generating electrical power. The hydraulic cylinder is drivenby a hydraulic fluid discharged from the hydraulic pump. The controllercontrols the power supply system.

A lead battery or a lithium-ion battery may be used as the battery.However, it is particularly preferred that a lead battery be used as thebattery.

When the electrical power is to be supplied to the motor-generatorand/or the AC motor and the electrical power regenerated by themotor-generator and/or the AC motor is used for charging, it ispreferred that the electrical power be supplied to the AC motor, andthat the electrical power regenerated by the AC motor be used forcharging.

It is preferred that the controller estimate the next operation of thehydraulic work device and/or the swing structure in accordance, forinstance, with the current operation by an operator and with measuredload, calculate the amount of electrical power to be regenerated by themotor-generator and/or the AC motor (preferably the AC motor) inaccordance with the estimated next operation, and set a target voltagefor the capacitor in accordance with the calculated amount of electricalpower.

It is preferred that the controller create a charge/discharge commandfor the capacitor in accordance with the calculated amount of electricalpower and set the target voltage for the capacitor in accordance withthe created charge/discharge command.

Further, the controller calculates the amount of electrical power inaccordance with the rotation speed of the AC motor, namely, inaccordance with a physical quantity that varies, for instance, with thecurrent operation by the operator and with the measured load.

The amount of electrical power can also be calculated by using theamount of energy.

It is particularly preferred that the controller compare the currentvoltage of the capacitor to the target voltage of the capacitor. If thecurrent voltage of the capacitor is higher than the target voltage ofthe capacitor, it is particularly preferred that the controller supplyelectrical power by discharging the capacitor.

Either the motor-generator or the AC motor is to be discharged. However,it is particularly preferred that the AC motor be discharged.

A process performed by the controller to estimate the next operation ofthe hydraulic work device or of the swing structure in accordance, forinstance, with the current operation by the operator and with themeasured load will now be described.

The hydraulic work device or the swing structure is driven inaccordance, for instance, with a lever operation performed by theoperator. In other words, when the amount of lever operation performedby the operator (a lever operation signal) is detected, the drive amountof the hydraulic work device or of the swing structure can be estimated.

The motion (drive) of a construction machine is substantially limited toa drive for construction work. Therefore, the next operation of thehydraulic work device or of the swing structure can be estimated inaccordance with the estimated drive (drive amount) of the hydraulic workdevice or of the swing structure. In accordance with the next operationof the hydraulic work device or of the swing structure, the amount ofelectrical power to be regenerated by the motor-generator or by the ACmotor is estimated.

To efficiently charge the battery or the capacitor with requiredelectrical power or efficiently discharge the battery or the capacitorfor acquiring required electrical power, it is necessary to control anelectrical storage ratio in advance.

As such being the case, the present invention predicts the nextoperation from the current operation of the construction machine, usesthe current information about a vehicle body to calculate the electricalpower for power generation or drive to be provided for the nextoperation by the AC motor or the motor-generator, calculates a targetvoltage for charging the battery and the capacitor in accordance withthe calculated electrical power, and performs a charge/discharge processin advance to obtain the target voltage.

Predicting the next operation from the current operation is peculiar toconstruction machines. In other words, it is necessary to consider notonly the drive of a travel section but also the drive of a swing sectionand hydraulic work device.

If, for instance, the current operation is a swing section power runningoperation, it is predicted that the next operation will be a swingsection regeneration operation. If, on the other hand, the rotationspeed of the engine is low for the current operation, it is estimatedthat an idling state prevails or that front work is under light loadconditions. In this instance, therefore, it is predicted that the nextoperation will be a power running operation.

In other words, the present invention calculates from the currentoperation the electrical power for power generation or for drive to beprovided for the next operation by the AC motor or the motor-generator.This makes it possible to calculate in advance the amount of electricalpower to be charged into or discharged from the capacitor or thebattery.

Advantages of the Invention

The present invention, which has been described above, provides aconstruction machine that is capable of charging electrical power intothe capacitor effectively with high energy efficiency in considerationof the drive of the swing section and hydraulic work device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a hydraulicexcavator to which an embodiment of the present invention is applied.

FIG. 2 is a diagram illustrating the system configuration of thehydraulic excavator.

FIG. 3 is a schematic diagram illustrating the control logic of acontroller.

FIG. 4 is a diagram illustrating the control logic of a capacitorcontrol section.

FIG. 5 is a diagram illustrating the relationship between swing leverpilot pressure, swing structure speed, energy E, capacitor voltage, andtime.

FIG. 6 is a diagram illustrating an exemplary power flow of a swingsection power running operation.

FIG. 7 is a diagram illustrating an exemplary power flow of a swingsection regeneration operation.

MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described withreference to FIGS. 1 to 7.

FIG. 1 is a diagram illustrating the configuration of a hydraulicexcavator (a representative example of a construction machine) to whichthe present embodiment is applied.

Referring to FIG. 1, the hydraulic excavator 2 includes a travelstructure 401 and a swing structure 402.

The travel structure 401 is driven by a travel hydraulic motor 33.

The swing structure 402 is driven by an AC motor 35 and formed turnablywith respect to the travel structure 401.

A cab seat 403 is disposed on one side of the front of the swingstructure 402 (e.g., on the left side as viewed from the back of thehydraulic excavator 2). A multijoint work section 400, which includes aboom 405, an arm 406, and a bucket 407, is disposed on the other side ofthe front of the swing structure 402 (e.g., on the right side as viewedfrom the back of the hydraulic excavator 2).

The boom 405, the arm 406, and the bucket 407 are respectively driven bya boom cylinder 32 a, an arm cylinder 32 b, and a bucket cylinder 32 c.

FIG. 2 is a diagram illustrating the system configuration of thehydraulic excavator.

Referring to FIG. 2, a system for the hydraulic excavator includes anengine 10, a motor-generator (M/G) 8, and a first inverter (INV) 9. Theengine 10 is controlled by an engine controller (ECU) 11. Themotor-generator (M/G) 8 is coupled to the engine 10 and to a hydraulicpump 31 and capable of generating electrical power. The first inverter(INV) 9 is connected to the motor-generator (M/G) 8 in order to controlmotive power generated by the motor-generator (M/G) 8.

During power running, the motor-generator (M/G) 8 assists the engine 10and drives the hydraulic pump 31, an air conditioner, and otheraccessory loads 16 connected to the engine 10. During regeneration, themotor-generator (M/G) 8 generates electrical power.

The generated electrical power (energy) is converted to a DC current bythe first inverter 9 and supplied to a power supply system 1. Thegenerated electrical power can be consumed by the swing structure 402.However, it is preferred that the generated electrical power be chargedinto a capacitor 13 in a power supply system 1.

The first inverter 9 converts DC power to AC power and converts AC powerto DC power.

The system for the hydraulic excavator also includes the hydraulic pump31 and a control valve 36. The control valve 36 drives a front workdevice 32 and the travel hydraulic motor 33 by controlling the flow of ahydraulic fluid supplied from the hydraulic pump 31.

The control valve 36 controls the flow of the hydraulic fluid suppliedfrom the hydraulic pump 31 to supply the hydraulic fluid to the boomcylinder 32 a, to the arm cylinder 32 b, to the bucket cylinder 32 c,and to the travel hydraulic motor 33.

The front work device (hydraulic work device) 32 includes the boomcylinder 32 a, the arm cylinder 32 b, and the bucket cylinder 32 c andis mounted on the swing structure 402.

The swing structure 402 is connected to the AC motor 35. The AC motor 35drives the swing structure 402.

Further, the AC motor 35 is connected to a second inverter (INV) 34. Thesecond inverter (INV) 34 drives the AC motor 35.

The swing structure 402 includes a speed reducer. The speed reducerdrives the swing structure 402 by increasing the speed of the shaftoutput of the AC motor 35. The AC motor 35 is a motor-generator. Duringpower running, the AC motor 35 acts as a motor (electric motor) togenerate a driving force. When the swing structure 402 is braked, the ACmotor 35 acts as a generator (power generator) to regenerate electricalpower.

The second inverter 34 converts DC power to AC power and converts ACpower to DC power.

The system for the hydraulic excavator further includes the power supplysystem 1.

The power supply system 1 includes a battery 12, the capacitor 13, afirst DC-DC converter 15 connected to the battery 12, and a second DC-DCconverter 14 connected to the capacitor 13. The battery 12 and thecapacitor 13 are electrical storage devices that store electricalenergy.

A lead storage battery having a voltage between 170 V and 360 V is usedas the battery 12. A lithium-ion battery may be used in place of thelead storage battery.

It is assumed that the capacitor 13 is an electric double layercapacitor and has a voltage of approximately 400 V and a capacitance ofapproximately 1000 F. When a 240 V lead storage battery is used as thebattery 12, it is necessary that the capacity of the capacitor 13 beapproximately 120 Ah although it depends on the work amount of theconstruction machine and the number of daily working hours.

The first DC-DC converter 15 and the second DC-DC converter 14 arestep-up/step-down choppers and act as a DC power converter forperforming power conversion between the inverters and electrical storagedevices by controlling its output in accordance with the amount ofelectrical power (energy) input into and output from the power supplysystem 1 so that a DC bus voltage is developed between the battery 12and the capacitor 13.

The first DC-DC converter 15 performs power conversion between thecapacitor 13, the first inverter 9, and the second inverter 34. Thesecond DC-DC converter 14 performs power conversion between the battery12, the first inverter 9, and the second inverter 34.

In other words, the power supply system 1 includes the electricalstorage devices (capacitor 13 and battery 12) and the power converters(first DC-DC converter 15 and second DC-DC converter 14) that performpower conversion between a direct current and an alternating current.

A controller 17 calculates the electrical current to be charged into ordischarged from the electrical storage devices (capacitor 13 and battery12) and controls the power converters (first DC-DC converter 15 andsecond DC-DC converter 14) in accordance with the result of thecalculation.

It should be noted that a current operation performed by an operator,body information, and load are input into the controller 17.

The amounts of lever, accelerator, and brake operations performed at thecab seat 403 are input as the operator's current operation. The speedand acceleration of the travel structure 401 and the information aboutthe swing structure 402, such as its turning speed and the attitudeinformation about a work section 400, are input as the body information.

The attitude information about the work section 400 can be derived fromthe amount of lever operation performed by the operator and defined asthe moment of inertia of the swing structure 402 in accordance with theamounts of operations performed relative to the boom, arm, and bucket.

Measured applied load is input as the load.

The controller 17 is connected to the engine controller (not shown), amotor controller (not shown), and a battery controller (not shown)through communication means to determine the flow of energy and controlthe engine 10, the motor-generator (M/G) 8, the AC motor 35, and thepower supply system 1 in accordance with various parameters such as theamount of operation performed by the operator, the electrical storagestatus of the power supply system 1, and the body information.

The present embodiment has been described on the assumption that ahydraulic excavator is a representative example of the constructionmachine. However, the present invention is also applicable to industrialvehicles and construction machines, which have an internal combustionengine (engine) and a power supply system. More specifically, thepresent invention can be applied not only to a hydraulic excavator butalso to a wheel loader, a forklift, and the like.

The present embodiment uses a power supply system that includes abattery (lead storage battery) and a capacitor. However, as the leadstorage battery suffers a significant loss during charging, regenerativeenergy cannot be efficiently recovered into the lead storage battery.Therefore, if the lead storage battery is repeatedly charged anddischarged with a large current, the lead storage battery tends tobecome increasingly degraded and shorten its life.

As such being the case, when the regenerative energy is to be chargedinto the power supply system 1, it is preferred that the capacitor 13 becharged wherever possible. This ensures that charging provides increasedenergy efficiency.

As described above, in a situation where the hydraulic excavator 2repeatedly turns or the work section 400 repeatedly moves up and down toperform front work, the present embodiment exercises management so as tooptimize the input and output of the capacitor 13.

Hence, the present embodiment grasps the status of the hydraulicexcavator in advance, calculates the amount of energy, predicts the nextoperation, determines the amount of energy to be charged into thecapacitor 13, and controls the DC-DC converters accordingly.

Consequently, the loss in the lead storage battery during regenerationcan be reduced to provide increased energy efficiency.

Although the turning operation of the hydraulic excavator 2 has beendescribed, the above is similarly applicable to traveling and excavatingoperations and to the other construction machines and industrialvehicles.

More specifically, the above is applicable, for instance, to forward andbackward movements, earth loading operations, and loading and unloadingoperations of a wheel loader or to traveling and lifting operations of aforklift.

As the operations of the construction machines are definite unlike thoseof automobiles, it is possible to grasp the next operation in advanceand exercise appropriate energy balance management.

FIG. 3 is a schematic diagram illustrating the control logic of thecontroller 17.

The amounts of lever, accelerator, and brake operations, the bodyinformation, the attitude information, and the load are input into thecontroller 17 as input information.

The amounts of lever, accelerator, and brake operations are determinedby a current operation performed by the operator at the cab seat 403.The body information includes the speed and acceleration of the travelstructure 401. The information about the swing structure 402 includesthe turning speed and the attitude information about a work section 400.

The attitude information about the work section 400 can be derived fromthe amount of lever operation performed by the operator and defined asthe moment of inertia of the swing structure 402 in accordance with theamounts of operations performed relative to the boom, arm, and bucket.

Measured applied load is input as the load.

A body control section 20 uses the above items of input information tocalculate outputs required for the hydraulic pump 31, the engine 10, theAC motor 35, and the motor-generator 26. These required outputs are usedby a hydraulic pump control section 21, an engine control section 22, anAC motor control section 23, and an M/G (motor-generator) controlsection 26, respectively. In accordance with the outputs required forthe aforementioned sections, the engine control section 22, the AC motorcontrol section 23, and the M/G control section 26 set an engine targetrotation speed We*, an AC motor torque command Tm*, and a motor torquecommand Tm2*, respectively, to drive the respective sections.

An energy calculation section 24 calculates the amount of electricalpower (regenerative energy) to be regenerated from the motor-generator(M/G) 8 and the AC motor 35.

The way of determining the regenerative energy for the swing structure402 will now be described.

The energy calculation section 24 calculates the sum of the potentialenergy Ep and kinetic energy Ev possessed by the hydraulic excavator 2as the regenerative energy.

The current operation by the operator, body information, and load thatare input into the controller 17 are used for the potential energy Epand kinetic energy Ev.

The kinetic energy Ev₁ for the swing structure 402 can be calculatedfrom a turning speed ωm [rad/s] by using Equation (1) below.

Ev ₁=(½)×I ₁ ×ωm ²  (1)

I₁ is the moment of inertia. The moment of inertia varies with theattitude of a front section of the hydraulic excavator that prevailsduring turning. Therefore, the amounts of lever operations relative tothe boom 405, the arm 406, and the bucket 407 or the moment of inertiaappropriate for pilot pressure should be defined in advance.

Similarly, the kinetic energy provided by the rotation of the engine 10can be calculated from an engine rotation speed ωe [rad/s] by using thefollowing equation.

Ev ₂ =K ₂×(½)×I ₂×ω²  (2)

I₂ is the moment of inertia of the engine and K₂ is a predeterminedconstant. K₂ is determined in accordance, for instance, with therotation speed of the engine 10 that prevailed Δt [s] earlier or with aboost pressure.

Hence, the kinetic energy Ev is calculated by adding Ev₁ to Ev₂(Ev=Ev₁+Ev₂).

The potential energy Ep can be calculated from Equation (3) below.

Ep=K ₃ ×St  (3)

St is the stroke [m] of the boom and arm that is required forregeneration in the front section. K₃ is a predetermined constant.

The sum of the potential energy Ep and kinetic energy Ev, which havebeen calculated as described above, is calculated as energy E. Theenergy E is regarded as the amount of electrical power to beregenerated.

In reality, the amount of electrical power to be regenerated is mostlycovered by the energy provided by the AC motor 35.

The rotation speeds and command values set by the body control section20, the information about the amounts of electrical power requested byvarious sections, and the regenerative energy calculated by the energycalculation section 24, which have been described above, are input intoan motion estimation section 25.

In accordance with the aforementioned items of information, the motionestimation section 25 estimates the next motions of the hydraulicexcavator's travel structure 401, front work device 32, and swingstructure 402.

In general, the work pattern of a construction machine, such as thehydraulic excavator 2, is basically predetermined. Therefore, the nextoperation can be easily estimated from the current operation. As regardsthe travel structure 401, for example, if the operator performs such anoperation as to increase a vehicle speed, it is estimated that the nextoperation will be performed to decrease the vehicle speed. As regardsthe work section 400, if the operator performs such an operation as toraise the bucket 407, it is estimated that the next operation will beperformed to lower the bucket 407. Further, when the swing structure 402turns to perform a power running operation, it is estimated that thenext operation will be performed to decrease the speed of turning. Inthe present embodiment, it is particularly preferred that the nextoperation of the swing structure 402 be estimated.

Although the present embodiment is described on the assumption that theswing structure 402 is electrified, the present invention is alsoapplicable to a case where the travel structure 401 and front workdevice 32 are electrified.

As regards the travel structure 401, for instance, travel-inducedkinetic energy Ev (=½×m×v₂) can be calculated from speed v [m/s]. When,for instance, the boom in the front work device 32 is electrified, theamount of electrical power to be regenerated can be determined bycalculating the potential energy, for example, from the weight m [g] ofearth placed in the bucket and from the height h [m] derived from theattitude information about the work section 400.

The amount of electrical power to be regenerated, which is calculated asdescribed above, is input into the motion estimation section 25 toestimate the current operation by the operator and next motion. Acapacitor control section 27, which charges the capacitor 13 withoutovercharging, sets a target voltage for the capacitor 13.

The way of setting a capacitor target voltage will now be described withreference to FIG. 4.

FIG. 4 is a diagram illustrating the control logic of the capacitorcontrol section.

As shown in FIG. 4, the capacitor control section 27 includes acapacitor charge/discharge command calculation section 40, a capacitorstatus detection section 41, and a capacitor target voltage setupsection 42.

The capacitor status detection section 41 detects a current capacitorvoltage, that is, the current voltage v₀ of the capacitor.

The current voltage V₀ detected by the capacitor status detectionsection 41 and the regenerative energy calculated by the energycalculation section 24 are input into the capacitor charge/dischargecommand calculation section 40. The capacitor charge/discharge commandcalculation section 40 then calculates a discharge command for thecapacitor.

The capacitor target voltage setup section 42 determines the capacitortarget voltage V_(c)* in accordance with the discharge commandcalculated as described above.

The discharge command for the capacitor, which is calculated by thecapacitor charge/discharge command calculation section 40, depends onthe amount of electrical power calculated by the energy calculationsection 24.

If, for example, the amount of electrical power is large, a chargecommand value is decreased or the discharge command value is increased.The reason is that the capacitor needs to be discharged beforehand toincrease its charge capacity in order for the regenerated electricalpower to be efficiently charged into the capacitor.

In other words, a great discharge command value should be set so as todischarge the capacitor beforehand. If, for instance, the enginerotation speed ωe is high during power running, it can be estimated thata heavy-load operation is being performed. In addition, as the capacitor13 cannot be charged, the charge command value is decreased. If a turboengine is being used, the load status can be predicted while consideringthe boost pressure as a parameter.

If, on the contrary, the amount of electrical power is small, acapacitor charge/discharge command is output so as to increase thecharge command value or decrease the discharge command value. When theamount of electrical power to be regenerated is small, the chargecapacity of the capacitor need not be increased. Therefore, a smalldischarge command value should be set. If, for instance, the enginerotation speed ωe is low, that is, the calculated amount of electricalpower is small, a small discharge command value should be set becausethe amount of regenerative power is small.

The capacitor target voltage setup section 42 sets the capacitor targetvoltage V_(c)* so that discharge occurs in compliance with the dischargecommand set by the capacitor charge/discharge command calculationsection 40 as described above. The capacitor target voltage V_(c)* iscalculated from the following equations by using the capacitor's currentvoltage V₀ detected by the capacitor status detection section 41 andusing the kinetic energy Ev and potential energy Ep, which constitutethe regenerative energy calculated by the energy calculation section 24.

V _(c) *=V0−E′  (4)

E′=(Kp×Ep+Kv×Ev)  (5)

Kp and Kv are predetermined constants.

As is obvious from Equation (4), control should be exercised to preparefor the next operation in such a manner that the capacitor targetvoltage V_(c)* decreases with an increase in the amount of electricalpower calculated by the energy calculation section and increases with adecrease in the amount of energy.

The capacitor control section 27 may correct the capacitor targetvoltage V_(c)* in accordance with the engine target rotation speed We*,the AC motor torque command Tm*, and the motor torque command T_(m2)*.

FIG. 5 is a diagram illustrating the relationship between swing leverpilot pressure, swing structure speed, energy E, capacitor voltage, andtime.

FIG. 5 shows the relationship between the swing lever pilot pressure,swing structure speed, energy E, capacitor voltage, and time thatprevails when the capacitor target voltage is controlled.

Referring to FIG. 5, when a swing lever is not operated, the kineticenergy is zero because the speed of the swing structure is zero.Therefore, the target voltage V_(c)* to be calculated from Equation (4)is set to be relatively high. When the swing lever is operated tomaximize the swing lever pilot pressure, the speed of the swingstructure increases accordingly. Thus, the kinetic energy in thisinstance is increased to decrease the target voltage V_(c)*. As regardsthe value of the target voltage, the constant Kv in Equation (5) shouldbe preset in compliance with the actual specifications for the hydraulicexcavator and in accordance with the maximum rotation speed of the swingstructure and with the maximum torque that can be output.

As the swing structure is turning at the maximum speed, the swingstructure decelerates when the swing lever is returned to neutral. Whendeceleration occurs, an output is generated in accordance with thekinetic energy to increase the target voltage V_(c)* for the capacitor.

The above explanation applies to a case where the swing lever isoperated to maximize the turning speed. However, the same also appliesto a case where the turning speed is low. When the turning speed is low,the kinetic energy is low. Therefore, the value of the target voltageV_(c)* for the capacitor, which is calculated from Equation (4), changesaccordingly. In the case of a power running operation, the value of thetarget voltage V_(c)* is increased. However, it is set to be lower thanfor the first turning operation shown in FIG. 5. As the speed of thesecond turning operation is low, the amount of energy that can beregenerated is small when the swing lever is returned to neutral toperform a regeneration operation.

Meanwhile, as for a battery control section 28 shown in FIG. 3, abattery target voltage V_(b)* is preset so that the SOC of the batteryapproaches to zero at the end of an operation of the constructionmachine. Therefore, the battery target voltage V_(b)* for the battery 12is controlled by merely discharging the battery 12 in accordance withthe preset battery target voltage V_(b)* and without regard to theamount of electrical power to be regenerated, which is calculated by theenergy calculation section.

Meanwhile, as the capacitor 13 is controlled in accordance with apreselected capacitor target voltage Vc* as mentioned earlier, thepresent invention discharges the capacitor 13 in advance. This controlprocess is performed to control the capacitor voltage and will now bedescribed with reference to FIG. 3.

The motion estimation section 25 exercises control so that the amount ofelectrical power to be discharged to obtain the capacitor target voltageV_(c)*, which is set by the capacitor target voltage setup section 42,is preferentially discharged to the AC motor 35. In this instance, thedischarge occurs to provide an output requested by each section, whichis calculated by the body control section 20, and the motion estimationsection 25 determines a power flow, that is, the flow of electricalpower supply to each section.

FIG. 6 shows that the electrical power stored in the capacitor ispreferentially discharged to the AC motor 35 in accordance with powerflows, which are determined by the motion estimation section 25, when apower running operation is performed by a swing section including theswing structure 402, the AC motor 35, and the second inverter 34.

When the amount of electrical power to be discharged from the capacitor13 covers the entire power Ps required by the swing section, varioussections are controlled in accordance with a power flow indicated by(1).

If the entire energy cannot be provided by the capacitor 13 due, forinstance, to a low voltage of the capacitor 13, the electrical powergenerated by the motor-generator (M/G) 8 can also be used in accordancewith power flows indicated by (1) and (2).

Further, if, in the battery control section 28, the battery is in a highSOC or the output of the engine is insufficient, control may beexercised to drive the AC motor 35 by using battery power in accordancewith a power flow indicated by (3).

All the above-mentioned power flows are based on the status of thecapacitor 13 and of the battery 12. As described earlier, decision ismade so as to comply with charge/discharge command values calculated bythe capacitor control section 27 and the battery control section 28.

The electrical power to be supplied to various sections is controlled inaccordance with the power flows determined as described above.Accordingly, a torque command for the motor-generator (M/G) 8 and atorque command for the AC motor 35 are issued.

Exemplary power flows for a case where the capacitor is to be charged,as described above, with electrical power regenerated from the swingsection and from the motor-generator after the capacitor 13 isdischarged in advance will now be described with reference to FIG. 7.

When the capacitor 13 can be charged, the various sections arecontrolled in accordance with a power flow indicated by (1). However,when a swing section power running operation is being performed, frontwork may be performed together with a turning operation as a compositeoperation. If, in such an instance, the amount of electrical powerregenerated by turning is large relative to a charge command for thecapacitor, such electrical power may be used as the output of the pump31 by driving the motor-generator (M/G) 8 to assist the engine withenergy regenerated by a swing motor in accordance with power flowsindicated by (1) and (2).

Further, if the charge command is output due to a low charge state ofthe capacitor 13, the electrical power generated by the motor-generator(M/G) 8 can also be used in accordance with power flows indicated by (1)and (3). In each case, the power flows are determined in such a mannerthat the capacitor target voltage V_(c)* for the capacitor is obtained.

The foregoing description deals with a case where a swing structurepower running operation or a regeneration operation is performed.However, the method of control provided by the power supply system 1according to the present embodiment is also applicable to a case wherethe other operations, such as a traveling operation and a front sectionboom operation, are electrified.

The present invention relates to a construction machine having an engineand a power supply system and is applicable, for instance, to ahydraulic excavator, a wheel loader, and a forklift.

DESCRIPTION OF REFERENCE NUMERALS

-   1 . . . Power supply system-   8 . . . Motor generator (M/G)-   9 . . . First inverter-   10 . . . Engine-   12 . . . Battery-   13 . . . Capacitor-   14 . . . Second DC-DC converter-   15 . . . First DC-DC converter-   17 . . . Controller-   32 . . . Front work device-   33 . . . Travel hydraulic motor-   34 . . . Second inverter-   35 . . . AC motor-   36 . . . Control valve-   400 . . . Work section-   402 . . . Swing structure

1. A construction machine comprising: an engine; a hydraulic pump thatis driven by the engine; a motor-generator that is coupled to the engineand to the hydraulic pump and capable of generating electrical power; ahydraulic work device that is driven by a hydraulic fluid dischargedfrom the hydraulic pump; a swing structure on which the hydraulic workdevice is mounted; an AC motor that drives the swing structure; a powersupply system having a capacitor that supplies electrical power to themotor-generator and/or to the AC motor and that becomes charged withelectrical power regenerated by the motor-generator and/or by the ACmotor; and a controller that controls the power supply system; whereinthe controller includes an motion estimation section that estimates thenext motion of the hydraulic work device and/or the swing structure inaccordance with the current operation performed by an operator, anenergy calculation section that calculates the amount of electricalpower to be regenerated by the motor-generator and/or by the AC motor,in accordance with the estimated next operation, and a capacitor controlsection that sets a target voltage for the capacitor in accordance withthe calculated amount of electrical power.
 2. The construction machineaccording to claim 1, wherein the capacitor control section includes acapacitor status detection section that detects the current voltage ofthe capacitor, a capacitor charge/discharge command calculation sectionthat calculates a charge/discharge command for the capacitor inaccordance with the calculated amount of electrical power and with thedetected current voltage, and a capacitor target voltage setup sectionthat sets a target voltage for the capacitor in accordance with thecalculated charge/discharge command.
 3. The construction machineaccording to claim 1, wherein the capacitor control section corrects thetarget voltage for the capacitor in accordance with the rotation speedof the engine, a torque command for the AC motor, and/or a torquecommand for the motor-generator.
 4. The construction machine accordingto claim 1, wherein the controller includes a body control section thatcalculates the amount of electrical power to be requested from the swingstructure in accordance with the current operation performed by theoperator.
 5. A construction machine comprising: a power supply systemhaving a capacitor that supplies electrical power to an AC motor fordriving a swing structure and that becomes charged with electrical powerregenerated by a motor-generator and/or by the AC motor, the swingstructure being a structure on which the motor-generator and/or ahydraulic work device are mounted, the motor-generator being coupled toan engine and to a hydraulic pump and capable of generating electricalpower, the hydraulic work device being driven by a hydraulic fluiddischarged from the hydraulic pump; and a controller that controls thepower supply system; wherein the controller estimates the next operationof the hydraulic work device and/or the swing structure in accordancewith the current operation performed by an operator, calculates theamount of electrical power to be regenerated by the motor-generatorand/or by the AC motor in accordance with the estimated next operation,and sets a target voltage for the capacitor in accordance with thecalculated amount of electrical power.
 6. The construction machineaccording to claim 5, wherein the controller calculates acharge/discharge command for the capacitor in accordance with thecalculated amount of electrical power and sets the target voltage forthe capacitor in accordance with the calculated charge/dischargecommand.
 7. The construction machine according to claim 5, wherein thecontroller calculates the amount of electrical power in accordance withthe rotation speed of the AC motor.
 8. The construction machineaccording to claim 5, wherein the controller compares the currentvoltage of a capacitor to the target voltage for the capacitor, and ifthe current voltage of the capacitor is higher than the target voltagefor the capacitor, supplies electrical power stored in the capacitor tothe AC motor that drives the swing structure.